The present invention relates to semiconductor laser devices and, in particular, to a semiconductor laser device capable of realizing high power, high reliability and long life, as well as its manufacturing method, and further to an optical disc reproducing and recording apparatus.
In recent years, there have been being achieved developments in AlGaAs-based semiconductor laser devices aimed at higher power and longer life by implementing an Al-free (Al-absent) quantum well structure (well layer and barrier layer). This is because the presence of Al at an oscillator end face would cause a surface level to occur at the oscillator end face, making catastrophic optical damage (COD) to be liable to occur, which is disadvantageous for high power, long life and high reliability.
For example, Japan Journal of Applied Physics Vol. 38 (1999) pp. L387-L389 reports a semiconductor laser device having an Al-free active region. This semiconductor laser device, as shown in FIG. 11, is made up by stacking on a GaAs substrate 301, one after another, a GaAs buffer layer 302, an Al0.63Ga0.37As lower cladding layer 303, an In0.49Ga0.51P lower guide layer 304, an In0.4Ga0.6P barrier layer 305, an In0.13Ga0.87As0.75P0.25 well layer 306, an In0.4Ga0.6P barrier layer 307, an In0.49Ga0.51P upper guide layer 308, an Al0.63Ga0.37As upper cladding layer 309 and a cap layer 310.
Generally, there is a difference in optimum growth temperature between an Al-free semiconductor layer and an AlGaAs-based layer. For example, InGaAsP or GaAsP or the like is low in growth temperature, as compared with AlGaAs-based materials. Therefore, in the case where an AlGaAs-based layer is stacked after the stacking of an Al-free semiconductor layer, it is necessary to interrupt the crystal growth after the stacking of the Al-free semiconductor layer, and then elevate the temperature before making the AlGaAs-based layer grown.
Unfortunately, since the Al-free semiconductor layer would remain exposed as a topmost surface during the interruption of crystal growth, temperature elevation would cause P to be desorbed (reevaporated), so that the Al-free semiconductor layer-AlGaAs-based layer interface would become larger in roughness.
To prevent this problem, the above-mentioned conventional semiconductor laser device is so designed that the Al-free semiconductor layer and the AlGaAs-based layer are grown continuously at a constant temperature. This provides an advantage that the crystal growth is not interrupted, thus preventing the Al-free layer from being exposed at the topmost surface during an interruption.
However, in the semiconductor laser device of the prior art as described above, the growth temperature for the continuous crystal growth of the AlGaAs-based layer is 720xc2x0 C., which is a higher temperature for the Al-free semiconductor layer. It is said that the optimum growth temperature for InGaAsP is about 650xc2x0 C, for example. As a result, even with the continuous growth, there has been left a problem that P is likely to be desorbed at the growth temperature of 720xc2x0 C., resulting in a roughened surface. This problem has had causal connections with increased deterioration and worsened reliability of the semiconductor laser device.
Accordingly, an object of the present invention is to provide a semiconductor laser device, as well as its manufacturing method, in which, for example, a non-P-based semiconductor layer containing no P as a principal ingredient is stacked on a quantum well active layer which is an Al-free P-based semiconductor layer stacked on a GaAs substrate and containing P, the semiconductor laser device having high power, high reliability and long life achieved by improving the crystallinity of the interface between the P-based semiconductor layer and the non-P-based semiconductor layer, and further to provide an optical disc reproducing and recording apparatus using the semiconductor laser device.
In order to achieve the above object, according to the present invention, there is provided a semiconductor laser device including, on a GaAs substrate, at least a first-conductive-type lower cladding layer, a lower guide layer, a quantum well active layer composed of at least one well layer and at least two barrier layers, an upper guide layer and a second-conductive-type upper cladding layer, one on another, wherein
at least one layer out of the plurality of layers is a P-based layer formed of group III-V compound semiconductor containing P as a group V element, and a layer adjoining this P-based layer is an As-based layer formed of group III-V compound semiconductor containing not P but As as a group V element, and
roughness of an interface between the P-based layer and the As-based layer is not more than 20 xc3x85.
Herein, the first conductive type refers to n type or p type, where if the first conductive type is n type, the second conductive type is p type; and if the first conductive type is p type, the second conductive type is n type.
With this arrangement, the roughness of the interface between the P-based layer and the As-based layer (non-P-based layer) is 20 xc3x85 or less. Therefore, the crystallinity of the interface is improved, so that a semiconductor laser device of high reliability, long life and high power can be obtained.
In one embodiment, the interface between the P-based layer and the As-based layer is disposed between the upper cladding layer and the lower cladding layer.
In this embodiment, the interface with its crystallinity improved is in a region where light is confined. Thus, a semiconductor laser device of high reliability, long life and high power can be obtained.
Also, in one embodiment, the P-based layer is formed of InGaAsP, InGaP, GaAsP or AlGaInP.
Also, in one embodiment, the As-based layer is formed of GaAs, AlGaAs, AlAs, InGaAs or AlGaInAs.
Also, in one embodiment, the well layer is the P-based layer.
In this embodiment, since the well layer is the P-based layer, the roughness of the interface of the well layer is 20 xc3x85 or less, the crystallinity of the interface concerning the quantum well active layer, i.e., within an active region is improved. Accordingly, a semiconductor laser device of high reliability, long life and high power can be obtained.
Also, the barrier layer is the P-based layer.
In this embodiment, since the barrier layer is the P-based layer, the roughness of the interface of the barrier layer is 20 xc3x85 or less, the crystallinity of the interface concerning the quantum well active layer, i.e., within an active region is improved. Accordingly, a semiconductor laser device of high reliability, long life and high power can be obtained.
Also, in one embodiment, both the well layer and the barrier layers are P-based layers and both the upper guide layer and the lower guide layer are As-based layers.
In this embodiment, the crystallinity of the interface between the quantum well active layer, i.e., an active region and the upper and lower guide layers is improved. Accordingly, a semiconductor laser device of high reliability, long life and high power can be obtained.
Also, in one embodiment, both the well layer and the barrier layers are formed of InGaAsP and both the upper guide layer and the lower guide layer in adjacency to the barrier layers are formed of AlGaAs.
In this embodiment, since both the well layer and the barrier layer are formed of InGaAsP, the crystallinity of the interface concerning the quantum well active layer, i.e., within an active region is improved. Further, since both the upper guide layer and the lower guide layer are formed of AlGaAs and the barrier layer is formed of InGaAsP, carrier overflow can be sufficiently suppressed by the energy (Ec) of conduction band and the energy (Ev) of valence band of AlGaAs while the reliability is ensured by preventing AlGaAs of the upper and lower guide layers from adjoining the well layer that involves occurrence of light-emission recombination, by virtue of the fact that the barrier layer formed of InGaAsP can be interposed between the upper and lower guide layers both formed of AlGaAs and the well layer. Accordingly, a semiconductor laser device of high reliability, long life and high power can be obtained.
Also, in one embodiment, Al composition ratio in the group III of the upper guide layer and the lower guide layer is not less than 0.20.
In this embodiment, since Al composition ratio in the group III of the upper guide layer and the lower guide layer is not less than 0.20, there can be obtained with a good balance a conduction-band energy difference (xcex94Ec) and a valence-band energy difference (xcex94Ev) with respect to the well layer of the quantum well active layer, so that the carrier overflow can be sufficiently suppressed. Accordingly, a semiconductor laser device of higher reliability, longer life and higher power can be obtained.
Also, in the semiconductor laser device of one embodiment, emission wavelength is not less than 760 nm and not more than 800 nm.
According to the present invention, there is provided a method for manufacturing a semiconductor laser device including, on a GaAs substrate, at least a first-conductive-type lower cladding layer, a lower guide layer, a quantum well active layer composed of at least one well layer and at least two barrier layers, an upper guide layer and a second-conductive-type upper cladding layer, one on another, wherein
at least one layer out of the plurality of layers is a P-based layer formed of group III-V compound semiconductor containing P as a group V element, the method comprising the steps of:
subjecting the P-based layer to crystal growth at a first growth temperature; and
above the P-based layer, starting growth of an As-based layer formed of group III-V compound semiconductor containing not P but As as a group V element at a growth temperature approximately equal to the first growth temperature, and thereafter furthering the growth while elevating the temperature to a second growth temperature.
In this method for manufacturing the semiconductor laser device according to the invention, above the P-based layer, i.e., just above the P-based layer or with a layer interposed on the P-based layer, the As-based layer is started to be grown at a growth temperature generally equal to the first growth temperature, which is a growth temperature of the P-based layer. Thereafter, the growth is furthered while the temperature is being elevated to the second growth temperature. Therefore, the roughness level of the interface between the P-based layer and the As-based layer can be lowered to 20 xc3x85 or less by reducing the desorption of P from the P-based layer. Thus, according to the present invention, it becomes implementable to fabricate a high-reliability, long-life, high-power semiconductor laser device.
Also, the method for manufacturing a semiconductor laser device of one embodiment further comprises the step of: before the growth of the As-based layer, making one layer or a plurality of layers of other As-based layer grown just above the P-based layer at a temperature approximately equal to the first growth temperature.
In this embodiment, one layer or a plurality of layers of other As-based layer are grown just above the P-based layer at a temperature generally equal to the first growth temperature, which is a growth temperature of the P-based layer. Therefore, the roughness level of the interface of the P-based layer can be lowered to 20 xc3x85 or less by reducing the desorption of P more preferably from the P-based layer. Thus, a semiconductor laser device of high reliability, long life and high power can be fabricated.
Also, in one embodiment, the first growth temperature is not less than 600xc2x0 C. and not more than 680xc2x0 C.
In this embodiment, since the P-based layer is grown at the first growth temperature of not less than 600xc2x0 C. and not more than 680xc2x0 C., which is more suitable for growth, the crystallinity becomes better so that the interface with the As-based layer is improved. Accordingly, a semiconductor laser device of high reliability, long life and high power can be fabricated.
Also, in one embodiment, the second growth temperature is not less than 700xc2x0 C. and not more than 780xc2x0 C.
In this embodiment, since the As-based layer is grown at the second growth temperature of not less than 700xc2x0 C. and not more than 780xc2x0 C., which is more suitable for growth, the crystallinity becomes better so that the interface with the P-based layer is improved. Accordingly, a semiconductor laser device of high reliability, long life and high power can be fabricated.
According to the present invention, there is provided an optical disc reproducing and recording apparatus which uses the semiconductor laser device as described above.
By virtue of using the semiconductor laser device having higher optical output than conventional, the optical disc reproducing and recording apparatus is enabled to read and write data even if the optical-disc rotating speed is enhanced over the conventional. Thus, the optical disc reproducing and recording apparatus of the invention is more convenient to operate by virtue of, in particular, its dramatic reduction in access time to the optical disc, which has hitherto been an issue in writing operation, as compared to conventional optical disc reproducing and recording apparatuses.